1. Technical Field
The present invention relates to stators for rotating electric machines that are used in, for example, motor vehicles as electric motors and electric generators.
2. Description of Related Art
There are known rotating electric machines that are used in motor vehicles as electric motors and electric generators. These rotating electric machines generally include a rotor and a stator. The stator includes an annular stator core and a stator coil. The stator core is disposed in radial opposition to the rotor and has a plurality of slots arranged in a circumferential direction thereof. The stator coil is comprised of a plurality of phase windings that are mounted on the stator core so as to be received in the slots of the stator core and be different in electrical phase from each other.
Japanese Patent Application Publication No. JP2011109894A (to be referred to as Patent Document 1 hereinafter) discloses a three-phase stator coil 140 as shown in FIG. 11. The stator coil 140 is comprised of a plurality of phase windings mounted on a stator core 130. Each of the phase windings is formed of a plurality of substantially wave-shaped electric wires; each of the electric wires is configured with an electric conductor having a substantially rectangular cross-sectional shape and an insulating coat that covers the outer surface of the electric conductor. Moreover, each of the phase windings includes a plurality of in-slot portions 151C and a plurality of turn portions 152. Each of the in-slot portions 151C is received in one of slots 131 formed in the stator core 130. Each of the turn portions 152 is located outside the slots 131 of the stator core 130 to connect one pair of the in-slot portions 151 C respectively received in two different ones of the slots 131. Moreover, each of the turn portions 152 includes an apex part 153 that is positioned at the center of the turn portion 152 in the extending direction of the turn portion 152 and extends in the circumferential direction of the stator core 130. Further, at the center of the apex part 153, there is formed a crank-shaped part 154 that is bent to radially offset the turn portion 152. Moreover, those turn portions 152 of the phase windings of the stator coil 140 which are located on a first axial side of the stator core 130 together constitute a first coil end part of the stator coil 140; those turn portions 152 of the phase windings of the stator coil 140 which are located on a second axial side of the stator core 130 together constitute a second coil end part of the stator coil 140.
More specifically, as shown in FIG. 11, the stator coil 140 is wave-wound on the stator core 130 so that each of the turn portions 152 connects a first in-slot portion 151 C arranged at the Nth layer counting from the radially inside in a first slot 131 and a second in-slot portion 151C arranged at the (N−1)th layer counting from the radially inside in a second slot 131 that is away from the first slot 131 by six slot-pitches, where N is a natural number greater than or equal to 2.
However, according to Patent Document 1, each of the electric wires forming the phase windings of the stator coil 140 is press-shaped (or bent), using shaping dies, into the substantially wave shape. That is, the apex parts 153 and crank-shaped parts 154 of the turn portions 152 that constitute the coil end parts of the stator coil 140 are formed by press-shaping. Therefore, at the crank-shaped parts 154 to which a high stress is applied during the press-shaping process, the insulating coats of the electric wires may be damaged, causing the electric conductors to be exposed from the insulating coats. Consequently, creeping discharge (or surface discharge) may occur between those portions of the phase windings of the stator coil 140 where the electric conductors are exposed, thus resulting in an insulation failure.
More specifically, as shown in FIG. 11, the crank-shaped parts 154 of the turn portions 152, where it is easy for exposure of the electric conductors to occur due to damage to the insulating coats during the press-shaping process, are circumferentially offset from one another by only one slot-pitch. Consequently, creeping discharge may occur in the stator coil 140.
Japanese Patent Application Publication No. JPH11164506A (to be referred to as Patent Document 2 hereinafter) discloses a three-phase stator coil 240 as shown in FIGS. 12-14. The stator coil 240 is comprised of a plurality of phase windings mounted on a stator core 130. Each of the phase windings is formed of a plurality of substantially U-shaped electric conductor segments; each of the electric conductor segments is configured with an electric conductor having a substantially rectangular cross-sectional shape and an insulating coat that covers the outer surface of the electric conductor. Moreover, each of the phase windings includes a plurality of in-slot portions 151C and a plurality of turn portions 152. Each of the in-slot portions 151C is received in one of slots 131 formed in the stator core 130. Each of the turn portions 152 is located outside the slots 131 of the stator core 130 to connect one pair of the in-slot portions 151C respectively received in two different ones of the slots 131. Moreover, each of the turn portions 152 includes an apex part 153 that is positioned at the center of the turn portion 152 in the extending direction of the turn portion 152 and extends in the circumferential direction of the stator core 130. Further, at the center of the apex part 153, there is formed a crank-shaped part 154 that is bent to radially offset the turn portion 152. Moreover, all the turn portions 152 of the phase windings of the stator coil 240 are located on the same axial side (i.e., the upper side in FIG. 12) of the stator core 130 and together constitute a coil end part of the stator coil 240.
More specifically, as shown in FIGS. 13-14, before being mounted to the stator core 130, each of the electric conductor segments is substantially U-shaped to have a pair of straight portions extending parallel to each other and a turn portion that connects ends of the straight portions on the same side. In forming the stator coil 240, the straight portions are axially inserted, from a first axial side (i.e., the upper side in FIG. 12) of the stator core 130, respectively into two slots 131 of the stator core 130 which are away from each other by six slot-pitches. Then, free end parts of the straight portions, which protrude outside the slots 131 on a second axial side (i.e., the lower side in FIG. 12) of the stator core 130, are twisted respectively toward opposite circumferential sides. Thereafter, each corresponding pair of distal ends of the twisted free end parts of the electric conductor segments are joined by, for example, welding. Consequently, in the resultant stator coil 240, those parts of the straight portions of the electric conductor segments which are received in the slots 131 of the stator core 130 respectively constitute the in-slot portions 151C of the phase windings of the stator coil 240; the turn portions of the electric conductor segments respectively constitute the turn portions 152 of the phase windings of the stator coil 240.
Moreover, according to Patent Document 2, the electric conductor segments forming the phase windings of the stator coil 240 are comprised of a plurality of large electric conductor segments 150A as shown in FIG. 13 and a plurality of small electric conductor segments 150B as shown in FIG. 14. The turn portions 152 of the large electric conductor segments 150A are larger than the turn portions 152 of the small electric conductor segments 150B. In each of the slots 131 of the stator core 130, there received four of the in-slot portions 151C of the electric conductor segments 150A and 150B in radial alignment with each other.
More specifically, as shown in FIG. 13, each of the large electric conductor segments 150A includes a first in-slot portion 151C arranged at the first layer (i.e., the innermost layer) in a first slot 131, a second in-slot portion 151C arranged at the fourth layer (i.e., the outermost layer) in a second slot 131 that is away from the first slot 131 by six slot-pitches, and one turn portion 152 that connects the first and second in-slot portions 151C. On the other hand, as shown in FIG. 14, each of the small electric conductor segments 150B includes a first in-slot portion 151 C arranged at the second layer in a first slot 131, a second in-slot portion 151 C arranged at the third layer in a second slot 131 that is away from the first slot 131 by six slot-pitches, and one turn portion 152 that connects the first and second in-slot portions 151C. Moreover, each joined-pair of the free end parts of the large and small electric conductor segments 150A and 150B respectively protrude from two slots 131 that are away from each other by six slot-pitches.
However, according to Patent Document 2, each of the electric conductor segments 150A and 150B forming the phase windings of the stator coil 240 is press-shaped (or bent), using shaping dies, into the substantially U-shape. That is, the apex parts 153 and crank-shaped parts 154 of the turn portions 152 that constitute the coil end part of the stator coil 240 are formed by press-shaping. Therefore, at the crank-shaped parts 154 to which a high stress is applied during the press-shaping process, the insulating coats of the electric conductor segments may be damaged, causing the electric conductors to be exposed from the insulating coats. Consequently, creeping discharge may occur between those portions of the phase windings of the stator coil 240 where the electric conductors are exposed, thus resulting in an insulation failure.
More specifically, as shown in the region F of FIG. 13, the crank-shaped parts 154 of the turn portions 152 of the large electric conductor segments 150A, where it is easy for exposure of the electric conductors to occur due to damage to the insulating coats during the press-shaping process, are circumferentially offset from one another by only one slot-pitch. Similarly, as shown in the region G of FIG. 14, the crank-shaped parts 154 of the turn portions 152 of the small electric conductor segments 150B, where it is easy for exposure of the electric conductors to occur due to damage to the insulating coats during the press-shaping process, are also circumferentially offset from one another by only one slot-pitch. Consequently, creeping discharge may occur in the stator coil 240.